Various non-contact material characterization techniques are known and are commonly used to measure semiconductor wafers. Non-contact material characterization techniques include: X-ray diffraction (“XRD”), eddy current measurements, and photoluminescence spectroscopy, among others. Photoluminescence spectroscopy, for example, is a technique wherein light is directed from a pump beam onto a sample, such as a semiconductor wafer. Such light may first be absorbed by the material and then dissipated, such as through emission of light (also known as “luminescence”). By measuring the intensity and spectral content of the luminescence by means of collection optics, various important material properties may be gleaned. Such properties revealed by photoluminescence include: determination of band gap, material quality (including the concentration of impurities and defects), composition of the different semiconductor layers, among many other properties. One useful way of analyzing the data may include plotting photoluminescence intensity as a function of wavelength. The full width at half maximum (“FWHM”) may then be measured and plotted.
Currently, such material characterization techniques are performed outside of the epitaxial growth apparatus in which the semiconductor wafers are formed. Commonly, the wafers are removed from the epitaxial growth apparatus and placed into cassettes of wafers. The cassettes are then cycled through, and the non-contact material characterization techniques are conducted on a wafer-by-wafer basis, with one wafer being tested at a time. This process can take a considerable amount of time.
Further adding to current processing times is the fact that typical processing apparatuses utilize a chamber referred to as a “load lock” in addition to the principal process chamber. A substrate, or a wafer carrier holding numerous substrates, is inserted into the load lock and brought to equilibrium with an inert atmosphere in the load lock compatible with the epitaxial growth process. Once the substrates are at equilibrium with the inert atmosphere in the load lock, a door between the load lock and the process chamber itself is opened, and the substrates are advanced into the process chamber. After processing, the substrates are removed from the process chamber through the load lock. Multiple handling into and out of the epitaxial growth apparatus takes considerable time, which in turn, slows the process.
With regard to photoluminescence techniques, for example, the wafers are typically placed on a stage and the pump beam and collection optics are either moved in a raster-scan or an outwardly spiraling pattern. That is, in the case of a raster-scan, the pump beam and collection optics are moved linearly across the surface in a first direction from one end of the wafer to the other. After fully scanning a first line across the wafer, the pump beam and collection optics are moved a small, incremental distance perpendicular to the first direction, and then they proceed to linearly scan across the surface parallel to and adjacent to the first line. This process is repeated until the entire surface of the wafer has been scanned. This technique is analogous to, for example, reading lines of text across the surface of a page from left to right and incrementally moving from the top line to the bottom. In the case of an outwardly spiraling pattern, however, the pump beam and collection optics begin scanning at the center of the wafer, and then they proceed to spiral outwardly from the center until the entire surface of the wafer has been scanned.
The above described prior art method of performing non-contact material characterization techniques can be very inefficient. Particularly in the case where multiple processes are to be performed on a group of semiconductor wafers, with material characterization occurring between each process, it can take a substantial amount of time to complete the overall process. Specifically, it can be very time consuming to first remove all of the wafers from the epitaxial growth apparatus after one process is completed, then to perform the testing on each wafer one at a time, and then to reseat the wafers on a wafer carrier and introduce the wafers to the same or a different apparatus for further processing.